In future cellular wireless access systems, as well as in some contemporary systems, a principle which may be used is the so called TDD principle, Time Division Duplex. In TDD systems, transmissions from the base stations to the user terminals in the cells, the “down link”, as well as transmissions from the user terminals to the base stations, the “up link”, are carried out on the same frequency, but with a division in time.
Due to the principle employed, in a TDD system there may be interference between uplink and downlink, as they are located on the same frequency. Thus, for example, a user terminal transmitting uplink to a base station may cause interference in another user terminal which is receiving in the downlink.
Similarly, a base station transmitting in the downlink to a user terminal may cause interference in another base station which is receiving traffic in the uplink. Such interference from other base stations may be very high, especially when there is line-of-sight between the base stations. Similarly, two terminals may be very close to each other, which is another case in which there may be very high interference.
One known way of addressing this problem is to synchronize and coordinate all base stations in the system so that all uplink and downlink periods occur simultaneously in all cells within the same area.
Additionally, so called “guard periods” may be inserted between up link and down link periods. Typically, the guard time at the transition from downlink to uplink is chosen to match the sum of the maximum roundtrip propagation delay in a cell and the time it takes for a terminal to switch from reception to transmission. Thus, due to the propagation delay, there is a delay before the terminal can receive the downlink data. In addition, the transmission timing can be controlled so that the terminal will start its transmission earlier to compensate for the propagation delay, in order for the data to be received within the uplink window at the base station.
The guard period at the transition from uplink to downlink, on the other hand, is typically chosen to match the time it takes for the base station to switch from reception to transmission, and the time it takes for a user terminal to switch from transmission to reception. Typically, the guard periods are given by a standard, or defined in the system profile, as is the case in some systems.
Thus, as described above, guard periods are chosen based on the propagation delays within the cell to avoid interference between uplink and downlink, and to allow the terminals to use consecutive uplink and downlink periods for transmission, and to allow the base station and the terminals to switch between uplink and downlink.
In the case that the guard periods are fixed, they are chosen when designing the system, and the same guard periods are used in all cells. If the guard periods are variable, they may be configurable in accordance to, for example, cell size.
However, despite the fact that the cells are synchronized, there may be high levels of base station to base station interference at the beginning or end of the uplink periods due to interference from distant base stations that are still on the air due to propagation delays, as well as interference from close base stations which due to synchronization errors start downlink transmission too early. Antenna down tilt and power control can be used to reduce the interference level, which however may come at the cost of reduced system performance.
Another way of addressing the problem of interference might be to introduce some form of channel coding. However, since the interference levels may be very high, both the radio frequency front-end of the base station, as well as the automatic gain control functions and the base band signal processing which performs demodulation need to be more advanced if the effects of RBS-RBS interference is to be addressed in this way.
Thus the guard times could be chosen not only based on the propagation delays within the cell, but also taking into account the propagation delays to base stations that can be “heard”, as well as synchronization errors. However, the required overhead in terms of guard periods may be difficult to determine for this, and for a worst case design, the overhead is applied to all cells in the network, despite the fact that all cells may not suffer from this base station to base station interference problem.